This is a study of the epidemiology and etiology of adolescent loneliness in a national sample of youth. The descriptive objectives include: (1) examining the multidimensionality of loneliness as experienced by American youth so as to establish the nature and meaning of this experience; (2) establishing the incidence of particular forms or dimensions of loneliness, both for the general adolescent population and for specific age, sex, class, and ethnic subpopulations; (3) describing patterns of change in the degree and form of loneliness across time for different age, class, race, and sex groups; and (4) describing the form and frequency of various responses to loneliness, i.e., coping strategies. The explanatory objectives involve a test of a general theoretical model specifying precipitating and predisposing factors in the emergence of loneliness. Basically, loneliness is viewed as the result of (1) weak or attenuated internal bonds; (2) weak or attenuated external bonds (deficits in social relationships); and (3) precipitating factors associated with developmental changes experienced during adolescence. The data include three national surveys with a youth panel (N equals 1734) completed in 1977, 1978, and 1979. Planned analyses include a number of multivariate taxonomic methods, comparative factor analyses, multiple regression, and age-cohort analyses employing t-tests and ANOVA techniques.